1. Field of the Invention
The present invention relates generally to optical fibers for use in data storage systems. More particularly, the present invention relates to low-birefringence optical fibers for use in magneto-optical data storage systems.
2. Background Art
In a magneto-optical storage system, using a magneto-optical (MO) recording material deposited on a rotating disk, information may be recorded on the disk as spatial variations of magnetic domains. During readout, a magnetic domain pattern modulates an optical polarization, and a detection system converts a resulting signal from optical to electronic format.
In one type of a magneto-optical storage system, a magneto-optical head assembly is located on a linear actuator that moves the head along a radial direction of the disk to position the optical head assembly over data tracks during recording and readout. A magnetic coil is placed on a separate assembly on the head assembly to create a magnetic field that has a magnetic component in a direction perpendicular to the disk surface. A vertical magnetization of polarity, opposite to that of the surrounding magnetic material of the disk medium, is recorded as a mark indicating a zero or a one by first focusing a beam of laser light to form an optical spot on the disk. The optical spot functions to heat the magneto-optical material to a temperature near or above a Curie point (a temperature at which the magnetization may be readily altered with an applied magnetic field). A current passed through the magnetic coil orients the spontaneous vertical magnetization either up or down. This orientation process occurs in the region of the optical spot where the temperature is suitably high. The orientation of the magnetization mark is preserved after the laser beam is removed. The mark is erased or overwritten if it is locally reheated to the Curie point by the laser beam during a time the magnetic coil creates a magnetic field in the opposite direction.
Information is read back from a particular mark of interest on the disk by taking advantage of the magnetic Kerr effect so as to detect a Kerr rotation of the optical polarization that is imposed on a reflected bear by the magnetization at the mark of interest. The magnitude of the Kerr rotation is determined by the material""s properties (embodied in the Kerr coefficient). The sense of the rotation is measured by established differential detection schemes and, depending on the direction of the spontaneous magnetization at the mark of interest, is oriented clockwise or counter-clockwise.
Conventional magneto-optical heads, while presently providing access to magneto-optical disks with areal densities on the order of 1 Gigabit/in2, tend to be based on relatively large optical assemblies which make the physical size and mass of the head rather bulky (typically 3-15 mm in a dimension). Consequently, the speed at which conventional magneto-optical heads are mechanically moved to access new data tracks on a magneto-optical storage disk is slow. Additionally, the physical size of conventional magneto-optical heads limits the spacing between magneto-optical disks. Because the volume available in standard height disk drives is limited, magneto-optical disk drives have, thus, not been available as high-capacity, high-performance commercial products. For example, a commercial magneto-optical storage device presently available provides access to only one side of a 130 mm double sided 2.6 ISO gigabyte magneto-optical disk, a 40 ms disk access time, and a data transfer rate of 4.6 MB/Sec.
N. Yamada (U.S. Pat. No. 5,255,260) discloses a low-profile flying optical head for accessing an upper and lower surface of a plurality of optical disks. The flying optical head disclosed by Yamada describes an actuating arm that has a static (fixed relative to the arm) mirror or prism mounted thereon, for delivering light to and receiving light from a phase-change optical disk. While the static optics described by Yamada provides access to both surfaces of a plurality of phase-change optical disks contained within a fixed volume, Yamada is limited by the size and mass of the optics. Consequently, the performance and the number of optical disks that can be manufactured to function within a given volume is also limited.
In applications that employ a polarization-maintaining fiber and Fabry-Perot (FP) laser to deliver polarized light from a source to a storage location, mode partition noise limits the available signal-to-noise ratio (SNR). Mode partition noise (MPN), in the form of broadband polarization fluctuations, is an intrinsic property of the FP laser that is manifest when a highly birefringent element is placed in its optical path. Polarization-maintaining optical (PM) fiber is, by design, very birefringent; therefore, MPN is very difficult to eliminate when PM fiber is utilized with a FP laser.
What is needed, therefore, is an optical data storage system that utilizes an optical fiber to convey light between a laser source and a storage location of an optical drive with a sufficient signal-to-noise ratio (SNR) and that allows an increase in the number of disks that can be placed within a given volume, as compared with conventional approaches. The improved optical head should preferably provide a high numerical aperture, a reduced head size and mass. Additionally, the optical head should improve upon conventional access to disk surfaces, disk drive access times, data transfer rates, optically induced noise, and ease of alignment and manufacture.
The present invention allows an increase in the number of storage disks that can be placed within any given volume by using low-birefringence optical fibers to transfer information to and from optical storage media. A high resonance frequency tracking servo device on a reduced profile head, in conjunction with the optical fibers, also provides improved access to storage media, improved disk drive access times, and improved data transfer rates.
An optical disk drive, in accordance with the present invention, utilizes various aspects of Winchester magnetic disk technology, for example, flying head technology. In another aspect of the present invention, a laser optics assembly directs light from an optical light source to an optical switch, which directs the light to one of a plurality of low-birefringence optical fibers coupled to one or more rotary arms, each of which support a flying optical head. Light is delivered through the optical fiber to a respective optical head for the purpose of reading and writing of data at a respective storage media with a focused optical spot. A reflected light signal from the storage media couples back through the optical head and optical fiber for subsequent processing. In one embodiment, the optical source of light comprises a Fabry-Perot (FP) laser. In another embodiment, the optical source of light comprises a stable single frequency laser source such as a distributed feedback (DFB) laser.
The optical path of the light delivered by the optical fiber is altered by a steerable micro-machined mirror mounted on the flying head. Track following and seeks to adjacent tracks are performed by rotating a central mirror portion of the mirror about an axis of rotation. A reflected light from the steerable micro-machined mirror is directed through an embedded micro-objective lens such as a GRIN (Gradient index) lens or a molded lens. A focused optical spot is scanned back and forth in a direction which is approximately parallel to the radial direction of the storage disk.
In another embodiment, track following and seeks to adjacent tracks may be performed with more than one storage media surface at a time by operating a set of steerable micro-machined mirrors independently from each other. A plurality of laser optics assembly are required in this particular embodiment.
In another aspect of the present invention, low-birefringence optical fibers are used to transfer information to and from magneto-optical storage disks. Due to inevitable stresses that are applied to the optical fibers, the SNR ratio of polarization information from the storage media may be degraded when conveyed by the optical fibers. The present invention provides an apparatus and method for increasing the SNR. In one embodiment, in-plane bend induced birefringence in low birefringence optical fibers is compensated for to increase the SNR. In another embodiment, both in-plane bend induced and out-of-plane bend induced birefringence are compensated for to increase the SNR. Out-of-plane bend induced birefringence may be compensated for by providing an optical polarization rotation element, which may comprise a rotatable xc2xd wave plate or a combination of a fixed xc2xc wave plate and a variable-phase wave plate. In-plane bending may be compensated by providing optical phase retardation of the reflected light. Phase retardation may be provided by an optical phase retardation element comprising, a liquid crystal retarder, a combination of a fixed xc2xc wave plate and a rotatable xc2xd wave plate, or a fixed xc2xc wave plate and rotatable polarizing beam splitter. In yet another embodiment, the SNR may be increased by providing an optical source of light comprising a modulated Fabry Perot or DFB laser.
In another aspect of the present invention, various noise reduction techniques are provided by substantially decreasing or eliminating spurious reflections (or the effects thereof) at the end faces of the optical fiber. These noise reduction techniques may be applied if the laser source is, for example, FP or DFB. In particular, spurious reflections (or the effects thereof) may be eliminated at the fiber front (launch) end face near the laser source and at the fiber back (head) end face near the storage media.
As similarly stated above, to eliminate the effects of the spurious reflection from the fiber front end face (i.e., launch end face), the laser source may be modulated at a particular frequency that depends on the length of the optical fiber. As a result, the spurious reflection from the fiber front end face is time-separated from the main signal-bearing beam returning from the storage media.
In another embodiment in accordance with the present invention, the spurious reflection from the fiber front end face is eliminated by coupling the fiber front end face to a material having a refractive index equal to the refractive index of the core of the optical fiber. The material may, for example, be formed from epoxy, fluid, or other suitable materials.
In another embodiment in accordance with the present invention, the spurious reflection from the fiber front end face is eliminated by coupling the fiber front end face to a cover slip having a refractive index equal to the refractive index of the core of the optical fiber. The cover slip may, for example, be formed from glass or other suitable materials.
In another embodiment in accordance with the present invention, the effects of a spurious reflection from the fiber front end face (i.e., head end face) are eliminated by polishing the fiber front end face at a particular angle with respect to an optical propagation axis. As a result, the main signal-bearing beam is spatially separated from the spurious reflection. In addition, a GRIN lens coupled to the fiber front end face may be polished at a similar angle with respect to the optical propagation axis in order to provide optimum fiber coupling efficiency.
In another embodiment in accordance with the present invention, the effects of a spurious reflection from the fiber back end face are eliminated by cleaving the fiber back end face at a particular angle with respect to the optical propagation axis. As a result of the angle cleave, the spurious reflection is not efficiently back-coupled through the fiber and is, therefore, effectively extinguished.
In another embodiment in accordance with the present invention, the spurious reflection from the fiber back end face is eliminated by coupling the fiber back end face to a fluid or epoxy having a refractive index equal to the refractive index of the core of the optical fiber.
In another embodiment in accordance with the present invention, the spurious reflections from the fiber back end face is eliminated by coupling the fiber back end face to a coreless or multi-mode fiber portion having a refractive index equal to the refractive index of the core of the optical fiber. The coupling of the fibers is carried out, for example, by fusion splicing.